14: Solutions

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Solutions play a very important role in many biological, laboratory, and industrial applications of chemistry. Of particular importance are solutions involving substances dissolved in water, or aqueous solutions. Quantitative measurements of solutions are another key component of this unit. Solutions can involve all physical states – gases dissolved in gases (the air around us), solids dissolved in solids (metal alloys), liquids dissolved in solids (amalgams – liquid mercury dissolved in another metal such as silver, tin or copper). This chapter will almost exclusively be concerned with aqueous solutions – substances dissolved in water.

14.1: Cameroon Tragedy
Lake Nyos is a crater lake in the Northwest Region of Cameroon, and is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity. A volcanic dam impounds the lake waters. A pocket of magma lies beneath the lake and leaks carbon dioxide into the water, changing it into carbonic acid. Nyos is one of only three known exploding lakes to be saturated with carbon dioxide in this way.
14.2: Solution Terminology
A solution is an even (or homogeneous) mixture of substances. A point should be made here that when a solution is said to have uniform properties throughout, the definition is referring to properties at the particle level. Solutions can be formed in a variety of combinations using solids, liquids, and gases. How much solute will dissolve in a particular solvent varies with each solute and is described qualitatively in a variety of ways.
14.3: Aqueous Solutions of Solids
In order for a solution to form, the intermolecular forces holding together the solute and solvent need to be broken and new interactions must be formed between them. Because of the need to disrupt intermolecular forces, temperature plays a large role in the solubility of various solutes.
14.4: Solutions of Gases in Water
The solubility of a gas in water is influenced by multiple factors. Temperature is one such factor, with gas solubility typically decreasing as temperature increases. This is one of the major impacts resulting from the thermal pollution of natural bodies of water. Another is pressure, with gas solubility increasing as pressure increases.
14.5: Solution Concentration Measurements
To define a solution precisely, we need to state its concentration: how much solute is dissolved in a certain amount of solvent. The mass/mass percent (% m/m) is defined as the mass of a solute divided by the mass of a solution times 100. Ratios involving the volumes may be used as well. When these ratios are multiplied by larger factors, units such as parts per million (ppm) and parts per billion (ppb) are obtained. These units may be used as conversion factors in addition to expressing concent
14.6: Molarity
Another way of expressing concentration is to give the number of moles of solute per unit volume of solution. Of all the quantitative measures of concentration, molarity is the one used most frequently by chemists. Molarity is defined as the number of moles of solute per liter of solution. The symbol for molarity is M or mol/L. Chemists also use square brackets to indicate a reference to the molarity of a substance.
14.7: Solution Dilution
In order to make a solution, we often take a sample of a concentrated stock solution and add solvent to it to dilute it to the desired concentration. The dilution equation provides an easy means of determining the amount of stock solution and solvent needed for a desired dilution.
14.8: Colligative Properties of Solutions
Freezing point depression and boiling point elevation are "colligative properties" that depends on the concentration of solute in a solvent, but not on the type of solute. The higher the concentration of solute, the more these properties will change. Another colligative property with important biological implications is osmotic pressure, caused by concentration differences in two solutions separated by a semipermeable membrane.
14.9: For Future Use
14.10: Exercises

Thumbnail Chapter 14: Colored solutions in round bottom flasks. (Raghav Bhasin via Unsplash)